The early universe was bathed with thermal neutrinos which decoupled from matter
at temperatures slightly above 1 MeV, around 1 s
after the big bang. Detection of these neutrinos, e.g.,
through inverse beta decay capture on tritium [1]} in the PTOLEMY
experiment [2]}, [3]}, remains a major challenge. The decoupled neutrinos carry information about the early
universe at an epoch much earlier than the time of photon decoupling. In addition, as neutrinos propagate through the universe, they acquire information about the intervening gravitational and magnetic fields they encounter en route to Earth.
We focus here on the time evolution of the helicity of primordial neutrinos and implications for their detection rates.
